Zirconium alloy tube for a nuclear reactor fuel assembly, and method for making same

ABSTRACT

PCT No. PCT/FR96/00137 Sec. 371 Date Jul. 29, 1997 Sec. 102(e) Date Jul. 29, 1997 PCT Filed Jan. 26, 1996 PCT Pub. No. WO96/24140 PCT Pub. Date Aug. 8, 1996A tube of zirconium-based alloy for constituting all or a portion of a cladding or guide tube for a nuclear fuel assembly. The tube is made of an alloy containing, by weight, 1.0-1.7% of tin, 0.55-0.80% of iron, 0.20-0.60% total of chromium and/or vanadium, and 0.10-0.18% of oxygen, with 50-200 ppm of carbon and 50-120 ppm of silicon. The alloy further contains only zirconium and unavoidable impurities, and it is completely recrystallized.

The present invention concerns zirconium-based alloy tubes for use innuclear reactor fuel assemblies. Tubes of that type are usable inparticular for constituting fuel rod cladding, for forming the externalportion of such cladding, or for forming guide tubes which receive therods of control clusters.

Cladding of that type is frequently constituted by tubes made from analloy known as "Zircaloy 4" which contains, in addition to zirconium,1.2% to 1.7% by weight of tin, 0.18% to 0.24% by weight of iron, 0.07%to 0.13% by weight of chromium and 0.10% to 0.16% by weight of oxygen. Anumber of alloys which are derived from those previous alloys have alsobeen proposed, in particular alloys in which the chromium is eithercompletely or partially replaced by vanadium and/or in which the oxygencontent exceeds that given above, with a corresponding reduction in thecontents of some of the other addition elements.

Particular qualities which are required in a tube for use as claddingare good resistance to corrosion by water at high pressure and at hightemperature, limited long term creep, long term retention of mechanicalproperties, limited expansion on irradiation and reduced sensitivity tolithium; in addition, these properties must be reproducibly obtainable,and the alloy must have metallurgical properties at the variousproduction stages (in particular rollability) which keeps the rejectionrate down to an acceptable value.

The behavior of Zircaloys on irradiation constitutes a factor which isinhibiting advances in operating conditions for nuclear reactors asregards increasing cycle time. This is mainly due to uniform corrosion.

A particular aim of the invention is to provide a tube with improvedcharacteristics which can be in the recrystallized state when good creepbehavior is required above all, or which can be in a metallurgicallystress-relieved state, which is more easily manufactured econom-cally towithin strict dimensional tolerances (in particular as regardscircularity errors) , and which is better as regards generalizedcorrosion.

For that purpose, there is provided a zirconium-based alloy tubecontaining, by weight, 1% to 1.7% of tin, 0.55% to 0.8% of iron, 0.20%to 0.60% in total of at least one element selected from chromium andvanadium, and 0.10% to 0.18% of oxygen, the carbon and silicon contentsbeing controlled and being respectively in the range 50 ppm to 200 ppmand in the range of 50 ppm to 120 ppm, the alloy further containing onlyzirconium and unavoidable impurities. The tube, in its final state, iseither stress-relieved or recrystallized depending on the requiredproperties.

Vanadium is essentially present in fine precipitates in the formZr(Fe,V)₂ ; this is also the case for chromium, which is present inprecipitates in the form Zr(Fe,Cr)₂.

A high Fe/(V+Cr) ratio, which may exceed 3/1, can further improveresistance to corrosion in a lithium-containing medium. As a generalrule, this ratio will be close to 2/1. It is generally preferable to useeither chromium alone, or vanadium alone rather than a combination ofthe two.

The precise composition selected from the above range will depend on theproperties which are to be prioritized. Usually, an alloy containing1.3% Sn, 0.60% Fe, 0.25% V or Cr, 0.14% O₂, 140 ppm C and 90 ppm Si willbe a good compromise.

The presence of vanadium reduces the fraction of absorbed hydrogen andimproves resistance to corrosion in an aqueous medium at hightemperature and high pressure, even in the event of localized boiling.

If one requirement is to reduce creep as much as possible during theinitial stage of reactor use, it may be advantageous to have a high tin,carbon, and/or oxygen content. A carbon content of more than 100 ppm isfavorable as regards creep; but above 200 ppm, expansion on irradiationbecomes large. The silicon content is "controlled" to take advantage ofits regulatory effect on structures and its favorable influence oncorrosion resistance.

A high value for the sum of the beta-producing elements (Fe+V+Cr)contributes to reducing the grain size of the metallurgical structure,which is a factor for good resistance to stress corrosion, ductilityafter irradiation, mechanical properties, and shaping. This sum isfrequently at least 0.70%.

The invention also provides a process for the production of an alloytube of the type defined above, comprising successively: casting aningot and forging to a solid bar; water quenching the bar after heating,generally by induction, in the β phase; optional annealing in the range640° C. to 760° C. (advantageously about 730° C.) to form the α phase;drawing a pierced billet to a tubular blank; optional annealing in αphase in the range 600° C. to 750° C. (advantageously about 650° C.);successive cold rolling steps to form tubes of decreasing thicknesses,with intermediate annealing steps in an inert atmosphere or in a vacuumat a temperature in the range 640° C. to 760° C., advantageously about730° C. for the first two steps and 700° C. for subsequent steps; and afinal annealing step in an inert atmosphere or in a vacuum at atemperature in the range 450° C. to 500° C. (advantageously about 485°C.) if a stress-relieved structure is required, or in the range 565° C.to 630° C. (advantageously about 580° C.) if a recrystallized structureis required. The set of heat treatments is advantageously such that theheat treatment parameter ΣA is in the range 10⁻¹⁸ to 10⁻¹⁶, ΣA g equalto the product of time t in hours multiplied by exp (-40000/T), T beingexpressed in Kelvins.

The first annealing step, after quenching, is advantageously carried outat 730° C.; the second, after extruding, is advantageously carried outat about 650° C.

The tube produced does not undergo any further heat treatment whichwould modify its metallurgical structure until the time it is used as acladding tube or a guide tube. However, it does undergo more surfacetreatment and is then examined. The surface treatment may in particularcomprise blast cleaning and film removal followed by rinsing. Thesurface treatment can be completed by polishing using a wheel. It ischecked conventionally, either visually, and/or using ultrasound and/orusing eddy currents.

Other characteristics will become more clear from the description ofparticular embodiments.

The following compositions found of interest:

    ______________________________________             COMPOSITIONS             I          II     III    ______________________________________    Tin        1.3          1.3    1.3    Iron       0.6          0.6    0.6    Vanadium   0.3          0.25   0    Chromium   0            0      0.25    Oxygen     0.12         0.14   0.14    Carbon     140          140    140    Silicon    90           90     90    ______________________________________

the other components being zirconium and impurities.

The starting alloy was in the form of an ingot. It was formed into a barby forging or rolling and, after heating to the β phase, was waterquenched at a controlled rate to bring it into the α region, for exampleat a cooling rate in the range 5° C. per second to 30° C. per seconduntil the temperature was less than about 800° C. After quenching,annealing was effected at a temperature of less than 800° C. to preventtransformation of the α phase into the β phase. Extrusion was carriedout after machining a tubular billet and heating to between 600° C. and700° C. The drawn blank, after undergoing any required annealing at atemperature of less than 800° C., then underwent the required number ofsuccessive cold rolling steps to bring it to the required thickness,with intermediate annealing steps carried out in argon, each for one tothree hours, to produce a suitable ΣA. In practice, four or five rollingsteps were generally carried out to produce solid cladding tubes ofconventional diameter and thickness. Finally, a final annealing step wascarried out in an inert atmosphere, at about 485° C. for one to threehours if a stress-relieved structure was required, or at about 580° C.for about two hours if a recrystallized structure was required.

The tests were carried out on samples to compare the alloys of theinvention containing different tin contents with Zircaloy-4 type alloys.

Generalized Corrosion

Tests were carried out on recrystallized samples in an autoclave, inwater and steam. The results are shown in Table I below.

                  TABLE I    ______________________________________                 Weight gain ΔP (mg/dm.sup.2)                   Water        Steam    ALLOY          350° C. - 210 days                                400° C. - 30 days    ______________________________________    1   Zr 0.6 Fe; 0.3 V                       29.2         26.4-38.5    2   Zr 0.6 Fe; 0.3 V; 0.5 Sn                       31           27.5    3   Zr 0.6 Fe; 0.3 V; 1.0 Sn                       32.2         30.4    4   Zr 0.6 Fe; 0.3 V; 1.5 Sn                       32           30.9        (invention)    5   Zircaloy 4     43.9-47.2    32    ______________________________________

The results obtained, in particular for alloy 4 which was in accordancewith the invention, show that an increase in the tin content from 0 to1.5% had no effect on generalized corrosion resistance in water andsteam.

Corrosion in a Lithium Medium and Creep Resistance

The influence of tin content on the corrosion resistance of Zircaloy 4type alloys in a medium containing lithium hydroxide was studied inwater containing 70 ppm of lithium at 360° C. The results are shown inTable II.

                  TABLE 2    ______________________________________    % Sn in Weight gain ΔP (mg/dm.sup.2)    alloy   50 days       100 days 150 days    ______________________________________    1.5     48            78       112    1.3     51            85       148    0.5     35            72       740    ______________________________________

The highly favorable influence of a high tin content (between 1.2% and1.5%) on the corrosion resistance in a lithium hydroxide medium wasobserved in alloys in accordance with the invention.

A high tin content was also shown to be favorable to the creepresistance of this alloy. Measurements of diametral creep ε_(D) at 400°C. over 240 hours at a pressure of 130 MPa gave the following values fora stress-relieved alloy:

    ______________________________________           Sn content (%)                    ε.sub.D (%)    ______________________________________           1.5 (invention)                    1.5           1.3 (invention)                    2           0.5      4.2    ______________________________________

The results obtained show a quasi-linear relationship between the Sncontent and the creep characteristics.

We claim:
 1. A zirconium-based alloy tube for constituting all or aportion of a cladding or guide tube for a nuclear fuel assembly,characterized in that the alloy contains, by weight, 1.0% to 1.7% oftin, 0.55% to 0.8% of iron, 0.20% to 0.60% in total of at least oneelement selected from chromium and vanadium, and 0.10% to 0.18% ofoxygen, the carbon and silicon contents being respectively in the range50 ppm to 200 ppm and in the range 50 ppm to 120 ppm, the alloy furthercontaining only zirconium and unavoidable impurities.
 2. A tubeaccording to claim 1, characterized in that the alloy is completelyrecrystallized.
 3. A tube according to claim 1, characterized in thatthe alloy is in a completely stress-relieved.
 4. A tube according toclaim 1, characterized in that the alloy contains about 1.3% of tin,0.60% of iron, 0.25% of vanadium or chromium, 0.14% of oxygen, 140 ppmof carbon and 90 ppm of silicon.
 5. A tube according to claim 1characterized in that the Fe/V ratio is close to 2/1, the alloy beingpractically free of chromium.
 6. A tube according to claim 1characterized in that the Fe/Cr ratio is close to 2/1, the alloy beingpractically free of vanadium.
 7. A tube according to claim 1characterized in that the sum of the iron and either vanadium orchromium content exceeds 0.7%.
 8. A process for producing an alloy tubein accordance with any one of claims 1 to 7, characterized in that itcomprises successively: casting an ingot and forging to a solid bar;quenching the heated bar to form the β phase; optional annealing in therange 640° C. to 760° C. to form the α phase; drawing a pierced billetto a tubular blank; optional annealing in the range 600° C. to 750° C.in α phase; successive cold rolling steps to form decreasingthicknesses, with intermediate heat treatments in an inert atmosphere orin a vacuum at a temperature in the range 640° C. to 760° C.,advantageously about 730° C., for the first two treatments and 700° C.for subsequent treatments; and a final annealing step in an inertatmosphere or under vacuum.
 9. A processing according to claim 8,characterized in that the final annealing step is stress-relievingannealing carried out in the range 450° C. to 500° C.
 10. A processaccording to claim 8, characterized in that the final annealing step isrecrystallization annealing carried out in the range 565° C. to 630° C.11. A tube of zirconium-based alloy for constituting at least an inneror outer portion of a cladding or guide tube for a nuclear fuelassembly, wherein said alloy contains, by weight, 1.0% to 1.7% of tin,0.55% to 0.8% of iron, 0.20% to 0.60% in total of at least one elementselected from a group consisting of chromium and vanadium, and 0.10% to0.18% of oxygen, and further contains controlled amounts of carbon andsilicon contents being respectively in a range of 50 ppm to 200 ppm andin a range of 50 ppm to 120 ppm, the alloy further containing onlyzirconium and unavoidable impurities.
 12. A tube according to claim 11,wherein the alloy is completely recrystallized.
 13. A tube according toclaim 12, wherein the alloy is completely stress-relieved.
 14. A tubeaccording to claim 11, wherein the alloy contains about 1.3% of tin,0.60% of iron, 0.25% of vanadium or chromium, 0.14% of oxygen, 140 ppmof carbon and 90 ppm of silicon.
 15. A tube according to claim 11,wherein the alloy is substantially free of chromium and has a Fe/V ratioof about 2/1.
 16. A tube according to claim 14, having a Fe/Cr ratio ofabout 2/1, the alloy being substantially free of vanadium.
 17. A tubeaccording to claim 14, wherein the sum of the iron, vanadium andchromium content exceeds 0.7%.
 18. A process for producing a tube of azirconium-base alloy, wherein said alloy contains, by weight, 1.0% to1.7% of tin, 0.55% to 0.8% of iron, 0.20% to 0.60% in total of at leastone element selected from a group consisting of chromium and vanadium,and 0.10% to 0.18% of oxygen, and further contains controlled amounts ofcarbon and silicon respectively in a range of 50 ppm to 200 ppm and in arange of 50 ppm to 120 ppm, the alloy further containing only zirconiumand unavoidable impurities, said process comprising the steps of:(a)casting an ingot and forging said ingot into a solid bar; (b) heatingthe solid bar and quenching it in β phase; (c) piercing the solid barand drawing it into a tubular blank; (d) carrying out successive coldrolling steps for obtaining decreasing thicknesses, with intermediateheat treatments in an inert atmosphere or in a vacuum at a temperaturein a range of 640° C. to 760° C.; and (e) carrying out annealing in aninert atmosphere or under vacuum.
 19. A process according to claim 18,wherein said heat treatments are carried out at about 730° C. for thefirst two treatments and 700° C. for subsequent treatments.
 20. Aprocess according to claim 18, wherein the final annealing step isstress-relieving annealing carried out in the range 450° C. to 500° C.21. A process according to claim 18, wherein the final annealing step isrecrystallization annealing carried out in a range of 565° C. to 630° C.22. A process according to claim 18, wherein the heat treatments as awhole are such that ΣA is between 10⁻¹⁸ and 10⁻¹⁶.
 23. A processaccording to claim 18, wherein the first annealing step is carried outat about 730° C.
 24. A process according to claim 18, wherein the secondannealing step after drawing is carried out at about 650° C.
 25. Aprocess according to claim 18, further comprising a step of annealing ina range of 600° C. to 750° C. in α phase after quenching.